(Note that FreeBSD's ar and ELF Tool Chain's copy have diverged, so a larger effort to review and incorporate changes from FreeBSD since 2009 or so is likely warranted.)
If you would like to refer to this comment somewhere else in this project, copy and paste the following link:
FreeBSD review for SYM64 support: https://reviews.freebsd.org/D18793
Committed to FreeBSD at https://svnweb.freebsd.org/changeset/base/346079
Upstream review, tests, and documentation still required.
(Note that FreeBSD's ar and ELF Tool Chain's copy have diverged, so a larger effort to review and incorporate changes from FreeBSD since 2009 or so is likely warranted.)